Polymer Chemistry

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Dynamic covalent chemistry in polymer networks: a mechanistic perspective Cite this: Polym. Chem., 2019, 10, 6091 Johan M. Winne,*a Ludwik Leibler*b and Filip E. Du Prez *a

The incorporation of dynamic covalent linkages within and between polymer chains brings new properties to classical thermosetting polymer formulations, in particular in terms of thermal responses, processing options and intrinsic recycling abilities. Thus, in recent years, there has been a rapidly growing interest in the design and synthesis of monomers and cross-linkers that can be used as robust but at the same time reactive organic building blocks for dynamic polymer networks. In this perspective, a selection of such chemistries is highlighted, with a particular focus on the reaction mechanisms of molecular network rearrangements, and on how various mechanistic proﬁles can be related to the mechanical and Received 20th August 2019, physicochemical properties of polymer materials, in particular in relation with vitrimers, the recently Accepted 16th October 2019 deﬁned third category of polymer materials. The recent advances in this area are not only expected to

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. DOI: 10.1039/c9py01260e help direct promising emerging polymer applications, but also point towards the need for a better funda- rsc.li/polymers mental understanding of chemical reactivity within a macromolecular context.

Introduction up of a (highly) cross-linked macromolecular network, wherein the covalent links between or within polymer chains can be Classical polymer materials are expected to be chemically inert dynamically exchanged via a chemical reaction. Such dynamic and thus to have a fixed molecular make-up. Normally, chemi- covalent polymer materials can thus undergo a net process cal reactivity within polymers is considered as a nuisance that corresponding to a molecular network rearrangement (MNR). This article is licensed under a needs to be avoided or controlled with stabilizers. This situ- In a broader context, with focus on polymer applications, such ation has changed over the last few decades by a growing inter- polymer systems have also been defined as covalent adaptable est in the area of dynamic polymer systems, designed to networks (CANs),7,8,10 as such materials can ‘adapt’ their Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. achieve functional characteristics such as self-healing, shape- macromolecular architecture in response to an external stimu- – memory, repairing, stimuli-responsiveness and adaptability.1 8 lus or trigger, despite their cross-linked nature. The response Whereas these desirable polymer dynamics can be influenced or triggered behavior of such polymer materials will not only in many ways and in diﬀerent contexts, such as in supramole- rely on the structural features of the polymer matrix but cular systems, one particular emerging strategy in this area has will also strongly depend on the rate and nature of the been to deliberately incorporate some degree of chemical reac- chemical bond exchanges within the polymer matrix. Thus, tivity into the polymer chains. Thus, many recent studies have the molecular mechanisms of these macromolecular rearrange- focused on the design of macromolecular matrices with ment reactions require close attention within the study – exchangeable, reversible or adaptable covalent bonds.1,2,7 9 In of such materials, as they can aﬀect the mechanical material light of these developments, chemical reactivity considerations properties. can no longer be ignored in considering the physical pro- Building a fundamental understanding of the variables that perties of such polymer materials, and often even take on a govern material properties of dynamic polymer networks or dominant role. rearranging polymer networks should provide further gui- In this perspective paper, we would like to focus on one par- dance for the design of new materials and for the selection ticular subclass of dynamic polymer materials, i.e. those made and optimization of suitable building blocks. Herein, we provide the reader with a selection of exchange chemistries that have been explored in this regard, with a focus on their aPolymer Chemistry Research Group and Laboratory for Organic Synthesis, reactivity and mechanistic profiles, pointing towards opportu- Department of Organic and Macromolecular Chemistry, Ghent University, nities both in material as well as chemical science. For recent Krijgslaan 281 S4-bis, B-9000 Ghent, Belgium. E-mail: [email protected], overviews of such chemistries with a focus on their diversity [email protected] bUMR Gulliver 7083 CNRS, ESPCI Paris, PSL Research University, 10 rue Vauquelin, and applications in polymers, we refer to the reviews by Zhao 11 12 75005 Paris, France. E-mail: [email protected] and Xie, and by Konkolewicz.

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Before discussing various types of network rearrangement ations on the facility with which various shapes and dimen- reactions, we first provide an overview of how synthetic sions can be given to such materials. Once this final shape is polymer materials have been categorized in terms of properties chemically fixed, the material becomes unprocessable. that are related to their macromolecular architecture, and then Vitrimer polymers are the third category of organic polymer – how the categories of thermoplastic and thermosetting poly- materials, introduced by Leibler and coworkers in 2011.14 16 mers13 have been refined and even expanded with a third cat- When heated, vitrimers behave like a viscoelastic liquid, but – egory, vitrimers,14 16 when taking into account dynamic when exposed to solvent they will only show a limited soluble covalent macromolecular architectures. fraction, related to defects and unattached chains. Vitrimers derive these properties from a covalent molecular network that can change its topology through molecular rearrangements, Thermoplastics, thermosets, and while preserving the total number of bonds in the network, i.e. vitrimers the network connectivity. When a vitrimer is heated above its glass (or melting) temperature, molecular rearrangements can At low temperatures, polymer materials are either glassy or par- take place and the topology of the network fluctuates. Thus, tially crystallized and behave like hard solids. Found on their network segments can diﬀuse even though the network pre- response to temperature or solvent, three categories of serves its connectivity while spanning the volume of the whole polymer materials can be distinguished. sample. In terms of material properties, the permanent nature Thermoplastic polymers consist of linear or branched of the network enhances the resistance to polymer wear pro- polymer chains. When enough thermal energy is provided, cesses such as dissolution, creep or solvent stress cracking. thermoplastic polymers melt and the chains will be able to The dynamic nature of the network facilitates processing and diﬀuse. Thus, when heated above a certain temperature level, reprocessing and brings recycling possibilities despite the thermoplastic polymers behave like a viscoelastic liquid and presence of permanent covalent cross-links.

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. flow. When cooled, the material solidifies again. This fact In a broader context, vitrimers are a very specific type of allows for the typical ‘plastic’ continuous processing options dynamic polymer networks. It is important to stress at this point that have made (thermo)plastics into the most popular the fundamental diﬀerence between permanent dynamic net- materials for rapid production of consumer goods of various works (i.e. vitrimers) and reversible dynamic networks. Reversible shapes and dimensions. dynamic polymer networks contain non-permanent covalent When exposed to good solvents (and heat), thermoplastic links or cross-links that are able to break and reform through polymers will completely dissolve. reversible chemical bonding reactions. Such systems have been – Because of their macromolecular character, molten thermo- studied theoretically and experimentally for decades,17 20 and plastics behave like viscoelastic liquids, especially when chains have recently received a great deal of renewed attention from

This article is licensed under a – are long and entangled. both academic and industrial research groups.1 12 Thermosetting polymers,13 on the other hand, when heated When heated, reversible polymer networks will undergo above their glass or melting temperature become softer, but topology fluctuations with fluctuations in overall connectivity, Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. they do not flow. Thermosetting polymers derive their endur- whereas in vitrimers, the postulated permanent nature of the ing and wide-ranging resistance to deformation from their network implies that the connectivity remains unchanged at (densely) cross-linked macromolecular structure, encompass- all temperatures and during the entire experimental time. This ing a macroscopic covalent network which spans the whole distinction is important because it will determine to some volume of the material. Segmental motions are hindered by extent the macroscopic properties that can be expected for connectivity and chain segments cannot diﬀuse. Thus, well such polymer networks. In a rearranging network with fluctu- above their glass transition temperature, thermosets exhibit ations in connectivity, i.e. changes in cross-link density, the rubber-like elasticity and behave like viscoelastic solids. When number of bonds within the polymer material is not constant, the network does not contain many defects such as dangling and the polymer network can be triggered (e.g. through appli- – ends or unattached chains, it exhibits mainly elastic responses cation of heat) to undergo a gel-to-sol transition,18 20 or to to deformations with little viscous losses. Conventionally, evolve from a classical thermosetting macromolecular architec- rubbers (or elastomers) are defined as thermosets with a glass ture to a classical thermoplastic one, and vice versa. These transition temperature below room temperature. materials can thus indeed be expected to either behave as a Even at high temperatures, when immersed into a good thermosetting polymer or as a thermoplastic polymer, depend- solvent, a thermosetting polymer material can only swell, and ing on the conditions that are applied. Thus, heat can trigger a only a limited fraction of the material, corresponding to loss of connectivity in a reversible dynamic network and chains unattached to the network, can be dissolved. promote thermoplastic flow, and similarly, the addition of Thermosets have an enhanced dimensional stability as they solvent can readily trigger a loss of connectivity and even a show resistance to polymer wear processes such as dissolution, complete dissolution after extensive loss of the network bonds. creep or solvent stress cracking. By necessity, thermosetting Thus, formally, reversible polymer networks are thermoplastic, materials need to be chemically synthesized (cross-linked) into as they flow when heated and can be completely dissolved in a their final shape. This so-called ‘curing’ process puts limit- good solvent.

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Polymer Chemistry Perspective

In the context of dynamic polymer networks, it should also through the network rearrangements. Theory and simulations, be noted that the three categories of polymer materials out- reported by Smallenburg, Leibler and Sciortino in 2013,22a seem

lined above are idealised systems. to indicate that, at least for vitrimers obtained through AfB2 con- On the one hand, these terms refer to combinations of pro- densation, at swelling equilibrium, the connectivity of the perties that are expected. Perfect thermosets cannot flow when swollen vitrimer network is changed with respect to the initial heated and should resist dissolution in a good solvent. Perfect vitrimer network, showing less defects and becoming more thermoplastic polymers should flow when heated and should perfect. Here, theory predicts that the soluble fraction and be soluble in a good solvent. Perfect vitrimers will flow when swollen network structure and connectivity will depend on heated but will also resist dissolution in a good solvent, even solvent concentration but are independent of temperature. In when heated. contrast to thermoplastics, the equilibrium cannot be shifted to On the other hand, and more fundamentally, the distinc- the solution state by heating. Thus, the features found in vitri- tion in these three classes should also be understood in con- mers defy the classification criteria of both thermosets as well nection to the underlying macromolecular architecture. Perfect as thermoplastics, justifying their position as the third category thermosets are made up by a permanent and static molecular of polymer materials that needs to be defined. network that may have some conformational freedom (or elasticity) but cannot undergo configurational changes. Perfect thermoplastics are non-covalently or reversibly linked polymer Rearrangement reaction pathways chains (or oligomers) that can diﬀuse by overcoming their weak intramolecular forces or bonds. Finally, vitrimers are per- Before discussing detailed examples of dynamic polymer net- manent molecular networks that can change their molecular works, it is worthwhile to first distinguish a few distinct configuration through segmental diﬀusion but without losing mechanistic scenarios. The exact architecture of macromolecu- network bond integrity. lar networks, including how cross-links and exchangeable

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Yet, in practice, distinctions between polymer categories bonds are distributed over the polymer backbone, and the rela- may be less evident to make. For example, high-molar-mass tive proximity of exchanging groups besides the availability highly entangled thermoplastics are characterized by very slow and mobility of catalytic species, are all factors that need to be stress relaxations, preventing processing by conventional taken into careful consideration. For clarity, however, it is methods. If such a high mass is desired, such polymers are good to first describe a generalized picture that makes abstrac- often synthesized and shaped by thermoset technologies and tion of these finer details and focuses on the nature of the the shape of the object has to be defined or controlled during bond exchange reaction itself. the synthesis. Still, such polymers can be soluble in a good Within a given macromolecular network, a rearrangement solvent. For dynamic polymer networks, the situation may be reaction can proceed via any of many possible mechanistic This article is licensed under a even more involved. For example, although reversible dynamic pathways, and the nature of the preferred or kinetically domi- networks with inherent fluctuations in cross-link density nant molecular network rearrangement (MNR) pathway will be should be categorized as thermoplastic, and not as vitrimers, important in considering the mechanical properties of a Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. it might in practice be diﬃcult to significantly shift the network, in particular with respect to distinction between the bonding/debonding equilibrium to a dissociated state, vitrimer (permanent) and reversible dynamic polymer net- depending on the reversible bond forming thermodynamics works described above. For our discussion, it is convenient to and reaction conditions. We will come back to this point later. consider network segments as if they were single molecules The possibility that permanent polymer networks with (Scheme 1), that undergo elementary rearrangement reactions exchangeable bonds could flow has been anticipated by just like small molecules would, making abstraction of specific Tobolsky already in 1956.21 However, the swelling behavior of polymer eﬀects such as long-range correlations or accessibility – such dynamic polymer networks is less obvious. Indeed, unlike of exchangeable bonds.17 20,22b Mechanistically, MNRs can for thermosets, the soluble fraction of vitrimers is expected not then be divided into two principal groups: only to contain chains that were initially not attached to the (i) Concerted (or single step) MNRs network, but also chains or segments that were detached (ii) Stepwise (or multistep) MNRs

Scheme 1 Molecular network rearrangements (MNR).

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Scheme 2 Diﬀerent types of molecular network rearrangements (MNRs) within covalent networks and their energy proﬁles.

In a concerted MNR (Scheme 2a), which is the least the prior association of two polymer chains, resulting in a new common reaction type, the new network bond can be formed covalent network bond, followed by an elimination step that at the same time as the old network bond is broken, without fragments another network bond. In the reactive (higher This article is licensed under a any kind of intermediate state, going through an ordered tran- energy) intermediate state, the cross-link density is thus tem- sition state. Polymer networks that exclusively undergo con- porarily increased, and the overall reaction rate will also certed rearrangements will thus always show the exact same depend on the proximity of the exchanging cross-links, as is Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. level of connectivity or cross-link density, as all possible also expected for concerted MNRs. In a strict sense, this situ- network topologies will have the same number of cross-links. ation does not meet the ideal requirements about the topology A concerted MNR can thus be considered as an ‘ideal’ scenario and connectivity fluctuations that are put forward for vitrimer to arrive at the formulated requirements for vitrimer polymers polymer materials, as the overall connectivity can be increased (vide supra). upon heating, as the equilibrium can shift to the endothermic The two most straightforward examples of non-concerted or side. In practice, however, this eﬀect will of course often be stepwise MNRs involve just two consecutive reaction steps: a negligible, resulting in a practically constant cross-link density discrete bond forming and a discrete bond cleavage step. The under a wide range of conditions. order with which these steps occur in a two-step MNR is quite In a polymer network that undergoes the bonding/debond- important. An associative stepwise MNR can be achieved ing reactions in reverse order, i.e. the dissociative stepwise through an addition/elimination pathway (Scheme 2b), MNRs (Scheme 2c), no association of polymer chain segments whereas the reversed order process, or an elimination/addition is required, and a polymer chain can undergo fragmentation pathway constitutes a dissociative stepwise MNR (Scheme 2c). by itself, forming a temporarily de-cross-linked intermediate The dissociative stepwise MNR has previously also been identi- state with the formation of reactive (higher energy) chain ends. fied as a ‘reversible addition’ bond exchange process,8 whereas A cross-link can then reform in a second step involving the associative stepwise MNR (also referred to as ‘reversible another polymer chain, resulting in the overall dissociative exchange’ or ‘associative exchange’) has so far been considered stepwise MNR reaction. This situation also does not meet the as an important prerequisite to arrive at vitrimer polymer ideal requirements about the topology and connectivity fluctu- network properties (i.e. flow when heated but have limited ations put forward for vitrimeric polymer materials, as the – solubility).14 16,23 overall connectivity can be decreased upon heating. Moreover, In a polymer network that undergoes an associative step- this decrease in connectivity is not limited by the conditional wise MNR (Scheme 2b), the rearrangement pathway involves involvement of another reactive species and can thus become

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Polymer Chemistry Perspective

quite extensive. The decross-linked intermediate state should MNRs and how this will influence the emergence of vitrimer also be entropically favored, rendering a significant shift in properties (as opposed to thermoplastic or thermosetting the bonding equilibrium more likely here than in associative ones). Previous descriptions, including those offered by our stepwise MNRs. research groups in the literature,22a,23 have not fully taken into It should be noted that many other, more complex multi- account the complete reactivity context. In fact, an ideal vitri- step MNR scenarios are probable and feasible. For example, a mer should only have MNRs via a concerted pathway catalysed version of all three scenarios in Scheme 2 is possible, (Scheme 2a), where the network exists in a single well-defined where a third species (as a catalytic additive) is involved. potential energy well, with the exact same degree of cross- Association of this catalyst is then an additional prerequisite linking, and dynamics are only possible through a fleeting for the exchange to proceed at an appreciable rate. This can be associated transition state that will have no lifetime. However, quite important, as the ratio of this catalyst compared to the as we shall discuss below, this situation is very rare. total number of dynamic cross-links will put an upper limit on The much more common stepwise MNRs, on the other the extent with which the equilibrium can be shifted to states hand, will by definition show energy profiles for polymer seg- with a different degree of connectivity (cf. Scheme 2b and c). ments with a higher energy (endothermic) state (as a separated One particular alternative pathway we would like to finally potential energy well), where the degree of connectivity can be point out here, is a dissociative stepwise MNR pathway that markedly different. The expectation then, within this concep- takes place through the mediation of a reactive small molecule tual framework, is that none of these materials can be ‘true’ as a mediator (formally speaking acting as a rearrangement vitrimers, as the overall degree of connectivity will always catalyst). In this fourth scenario (Scheme 3), polymer chains change upon heating, by shifting of the equilibrium to the are reversibly linked through a condensation reaction, uniting endothermic side. However, it should be clear that many situ- two chain fragments while eliminating a small molecule. As ations will be possible where highly dynamic MNRs can be long as the small molecule that is formed during condensation achieved in a wide temperature window, without necessitating

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. (most typically a molecule of water) remains present in the significant changes in the overall connectivity of the network. polymer matrix, the cross-links will remain reversible via a dis- Thus, many stepwise MNRs, and in particular the associative sociated intermediate state. In classical polymer synthesis ones, will result in clear vitrimer properties. This point will be terms, the dynamic process shown in Scheme 3 corresponds to addressed more explicitly later on. a ‘reversible condensation’ polymerisation.8 The mechanistic categories outlined above will allow a more systematic discussion of dynamic polymer networks that Associative stepwise rearrangements undergo rearrangement reactions. Examples of all four of the presented generalized mechanistic types have been reported in The most common scenario that can be expected to meet or at This article is licensed under a the literature, and below, we have selected illustrative cases. least approach ideal vitrimer requirements is found in This selection is not an exhaustive listing of dynamic covalent polymer systems that can exclusively undergo associative step- chemistries for each mechanistic type but has rather been made wise MNRs (Scheme 2b). In these systems, network decross- Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. based on the amount of available studies and characterisations linking is not possible, and the build-up of additional cross- of the chemistry underlying the rearrangement process. links can often be suppressed, even under conditions of very An important question we would like to address in this per- rapid bond exchanges, leading to only negligible fluctuations spective is the relationship between mechanistic profiles of in cross-linking density, while having very rapid fluctuations in network topology. The first truly associative bond exchanges were purposely implemented in polymer chemistry by Bowman and Cook, already in 2005, by their elegant design of cross-linked networks that undergo photoinduced radical addition–fragmentation reactions resulting in plasticity and a certain stress relaxation.24 Being chain mechanisms in nature, their overall rates critically depend on concentration of free radicals and on the relative reaction rates of many competing processes (e.g. initiation vs. termination rates). Radical addition and fragmentation reactions often proceed without barriers, operating at the diﬀusion limit with no real activation energy. They do not require thermal triggers, and do not adhere to the generic situations outlined in Scheme 2. Radical chemistry will thus not be explicitly considered in this perspec- – tive, and we refer readers to other dedicated reviews.7 12 In 2011, Leibler and coworkers purposely introduced an associative bond exchange reaction into a polymer network.14 Scheme 3 Small molecule mediated dissociative exchange. The underlying idea was to design a material that will flow

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Perspective Polymer Chemistry

when heated but would not be soluble owing to the permanent cosity profile typical of strong glass formers, not showing the nature of the network. This is a combination of properties that fragility that is classically associated with organic polymers. is common for inorganic materials like silica glasses but is not Without undergoing side reactions, the material also cannot found for organic polymer materials. Thus, they designed an show a sol-to-gel transition, as the overall cross-link density epoxy-based polyester resin that includes free hydroxyls, judi- does not decrease upon heating.22a The temperature depen- ciously placed next to ester linkages that make up the entire dence of the viscosity of these materials showed a good agree- polymer backbone (Scheme 4a). Introduction of a suitable ment with that observed for the chemical reaction rate, (Lewis) acid or base catalyst accelerated the dynamic inter- showing a connection between the dynamics of viscoelastic change of ester linkages between hydroxyl moieties via a very flow and the chemical dynamics.15,23 feasible addition/elimination reaction inside this polymer By changing the catalyst and its concentration, the mechan- network. This approach gave an organic material that can ical properties of polyhydroxy-polyester resins have been swell, but is completely insoluble in good solvents, even at high shown to be directly modified and controlled.15,23 Catalyst temperatures. Despite this insolubility, the material also dis- loading shows a linear relationship to reaction rate, if the plays full stress relaxation and flow, mediated by the rapid mechanism is first order in that catalyst. By keeping the cata- dynamic bond exchanges. These materials also showed a vis- lyst to hydroxyl ratio low, side reactions and extensive additional cross-linking upon heating can be minimised. Diﬀerent catalysts also result in other activation energies, which can be measured in situ by mechanical stress relaxation experiments. Thus, judicious choice of small amounts of additives that can act as a catalyst can drastically alter the mechanical properties of one and the same polymer matrix, which is a powerful design concept for polymer materials.

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. In 2014, Winne and Du Prez and coworkers introduced the dynamic covalent chemistry of enaminone-moieties derived from amines and β-ketoesters as a platform chemistry for the design of vitrimer materials.25 The resulting linkage can formally be considered as a vinylogous urethane, with a vinylic bond inserted in between the electron donating nitrogen and the electron-withdrawing ester moiety (Scheme 4b). This elec- tronic eﬀect makes vinylogous urethanes into thermo- dynamically stable linkages, very similar in nature to classical This article is licensed under a isocyanate-derived urethane cross-links. However, whereas normal urethanes do not readily undergo (catalyzed) addition/ fragmentation reactions because of their weak electrophilic Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. properties,26 the vinylogous urethanes are more reactive because of the Michael-type reactivity of the α,β-unsaturated carbonyl moiety. Another interesting feature of this chemistry is its resistance toward hydrolysis, as amines and β-ketoesters spontaneously undergo a condensation to form the enaminone, even in aqueous media. Another classical associative stepwise MNR bond exchange we want to draw attention to here is that between hydroxyls and siloxanes. In 2017, Guan and coworkers implemented siloxanes as cross-linkers for hydroxy-functionalised poly- 27 styrene backbones (Scheme 4c). The resulting high Tg networks could undergo rapid exchange reactions upon heating via an addition/elimination reaction of the silyloxy groups to the silyl ether linkages. Interestingly, the inclusion of basic amine moieties in the cross-linkers reduced the activation energy for the exchange reaction and accelerated the reaction as a covalently embedded or internal catalyst. Also in 2017, Nicolaÿ and Leibler reported the straightforward synthesis of cross-linked simple polyvinylic networks incorporating boron-heterocyclic cross-links (Scheme 4d).28 Dioxaborolanes are cyclic esters of boronic acids and a vicinal Scheme 4 Examples of associative stepwise MNR-based materials. diol, which spontaneously undergo a condensation upon

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Polymer Chemistry Perspective

bringing the two together. This ring formation is entropically favored by the expulsion of two molecules of water. The boron–oxygen bonds are very strong because of their significant π-bonding character between the oxygen lone pair and the unoccupied p-orbital on boron, reminiscent of a classical carbonyl bond. At the same time, the boron center remains highly electrophilic and readily forms tetrahedral intermediate adducts with a wide range of nucleophiles. The dynamic exchange with hydroxyls and diols has been used in boronic ester transesterification vitrimers developed by Guan and coworkers,29 but Nicolaÿ and Leibler found that the exchange can also happen as a metathesis reaction between two dioxaborolanes without the need for a free hydroxyl moiety, simplifying its implementation. The exchange mechanism of the metathesis is unclear, but it is likely a multistep process, initiated by the formation of a Scheme 5 Early examples of associative exchanges in polymer networks. zwitterionic adduct wherein alkoxide residues can be readily exchanged between the two boron centers, ultimately enabling a complete crossover of bonding partners – and thus the attached polymer chain portions – through a series revealing PDMS elastomers as an overlooked or forgotten class of addition/fragmentation reactions. The resulting MNR of dynamic materials (Scheme 5a). Siloxane transesterifica- profile is thus likely more complex than that shown in tions have since been used in the purposeful design of vitri- 27

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Scheme 2b, as multiple intermediate states are formed here, mers (vide supra: Scheme 4c). However, alkoxide-mediated wherein in total four bonds are broken and four new bonds dynamic PDMS networks are prone to depolymerisation by are formed, and alternative neutral intermediate species may very facile ‘backbiting’ and depolymerisation via a reverse ring be involved. The timing of these intermediate steps will be of opening polymerisation, severely limiting their potential as little importance, as all realistic reactive intermediates are vitrimer materials. expected to have a higher connectivity than that of the resting Likewise, classical polysulfide rubbers have been long states of the polymer networks. Interestingly, this chemical known to incorporate dynamic S–S linkages (Scheme 5b), platform allows the incorporation of associative dynamic since the seminal studies of hydrocarbon rubber vulcanizates exchanges without the need for a catalyst, but also without by Stern and Tobolsky in 1946.34,35 Although the exact This article is licensed under a the need for any other functionality except the dioxaborolane, exchange mechanism in vulcanized rubbers remains unclear which undergoes a metathesis with itself and is a quite stable to this day, the process is clearly associative in nature (first moiety. order kinetics in disulfide and in thiols) and does not involve Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. As a final example, we also want to point out the recent disulfide bond cleavage. Alternatively, a direct metathesis work of Bowman and coworkers on the dynamic exchange of exchange of disulfide cross-links via adduct formation has thioesters with free thiols (Scheme 4e).30,31 This transesterifi- also been postulated by Klumperman (not shown in cation is very rapid in the presence of base or acid catalysts, Scheme 5b).36 Given the prevalent associative pathway for di- even at room temperature. However, by cleverly combining this sulfide exchange, such dynamic cross-links have also found chemistry with a photobase, Bowman and coworkers obtained applications in the purposeful design of vitrimer materials, a thiol–ene based polymer network that can be switched as for example in the works of Odriozola.37,38 However, classi- between dynamic and non-dynamic states by including a cal polysulfide rubber formulations rarely show complete photobase, opening up a new concept for dynamic polymer stress relaxation,39 and do not flow when heated, possibly systems.30 related to the presence of permanent (thioether) cross-links From a historical perspective, it is interesting to point out and the limited dynamics within the overall molecular archi- that associative exchanges within polymers have been specu- tecture of these rubbers. lated upon and studied even in the early days of polymer For an overview of other associative exchange chemistries, chemistry (1940s–1950s). In 2012,32 McCarthy and coworkers we refer to recent reports.11,12,23 Among many reactions ‘rediscovered’ the classical work on chemical stress relaxations studied in this context, a particularly promising one seems to in PDMS elastomers reported by Osthoﬀ et al. in 1954,33 be one reported by Hillmyer and coworkers,26 demonstrating and contemporaneously studied by Tobolsky.21 McCarthy vitrimers based on transreactions (addition/elimination) of realized that the reported exchange reactions of silyl ethers hydroxyl moieties in polyhydroxy-polyurethanes and in poly- would have potential for modern self-healing dynamic hydroxy-polycarbonates.40 These results show that associative materials (Scheme 2b). The well-established alkoxide-mediated stepwise MNR can be designed into common and widely addition–elimination that can happen at PDMS chain ends popular polymer matrices, using chemical functions that are can indeed act as a dynamic rearrangement mechanism, widespread.

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Dissociative stepwise rearrangements

In the previous section, examples of stepwise associative polymer rearrangements have been discussed. These are all relatively recent additions to the collection of dynamic covalent chemistries that have been implemented in polymer networks, striving for vitrimer polymer materials. Stepwise dissociative bond exchanges (MNRs) are in fact much more common in polymers, and the main mechanism underlying the dynamics here is a ‘reversible addition’ process, or more precisely an elimination/addition process, in which a chain segment first fragments into two new chain ends (Scheme 2c). In fact, chain fragmentation is probably one of the most common chemical reactions within a polymer matrix, and it is also a leading mechanism of polymer degradation or material failure. However, typical bond fission reactions result in high energy radical or ionic fragments. Such highly reactive chain ends will usually be very prone to undergo secondary reactions, rendering the bond cleavage eﬀectively non-reversible, and leading to permanent damage to the material, although there are elegant examples developed by Bowman that have harnessed free radical reactivity for the development of covalent adaptable 24,41

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. networks. For a straightforward MNR, however, a fragmentation process should result in two relatively well-defined and robust chain ends, which do not readily undergo side reactions, as they should be able to recombine without permanent degradation of the polymer matrix or its material properties. Perhaps the most well-known example of a fragmentation that reliably produces closed shell, non-ionic fragments is the (retro)-Diels–Alder reaction (Scheme 6a–c). The concerted This article is licensed under a nature of the reaction implies that two Sigma bonds are Scheme 6 Examples of dissociative stepwise MNR-based materials. broken at the same time, which immediately transform into two stable pi-bonding arrays, embedded within the diene and Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. the dienophile. these small intrinsic thermodynamic preferences, substitution Interestingly, reversible Diels–Alder chemistry was first patterns of the bonding partners as well as the nature of the explored in polymers in industrial labs at DuPont,42 exploiting polymer matrix can have very pronounced eﬀects on this equili- the thermoreversible cycloaddition reaction between furans and brium, shifting it towards one end or the other. Mild heating in maleimides, as readily available bulk chemicals. Thus, already a solvent can also rapidly displace the equilibrium towards the in 1966, Craven described several reversibly cross-linked non-cross-linked state, and furan-maleimide networks are thus polymer resins showing a unique ‘post-formability’ very fragile towards dissolution by a solvent. (Scheme 6a). This chemistry has so far not found widespread In 2009, Lehn and coworkers developed self-healing poly- industrial applications, but interest for it has grown enormously mers by applying a room temperature reversible Diels–Alder in recent decades, since the seminal works of Hodge and reaction between fulvenes and highly reactive acrylate-type die- Gandini,43 and Wudl.44 While the bond-forming cycloaddition nophiles (Scheme 6b).46 The incorporation of such reactants of furans and maleimides is reversible, the forward reaction inside a polymer network requires some synthetic eﬀort, but typically does not occur at a significant rate at room tempera- network formation is straightforward, as the forward Diels– ture and also does not readily proceed to full completion upon Alder reaction readily proceeds within a matter of seconds at heating. Curing requires application of heat and is rarely com- room temperature (and takes several hours at −10 °C).47 Again, plete, especially under stoichiometric conditions (1 : 1 furan : the free energy of the reaction is very small, in the range of −8 − maleimide). The bond forming process is characterised by a to −15 kJ mol 1. Thus, already upon moderate heating − relatively high activation reaction barrier (>100 kJ mol 1)and (>50 °C), the equilibrium can be quickly and significantly shows only a low reaction free energy in the range of −8to shifted to the endothermic, de-cross-linked side. This dynamic − −15 kJ mol 1.45 The free energy of association between furan covalent polymer network thus actually approaches the charac- and maleimide is thus comparable to that of a hydrogen bond teristic properties of supramolecular networks that are held interaction, albeit one that is quite hard to break. In light of together by weak non-covalent interactions.

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In 2010, Barner-Kowollik and coworkers used simple dithio- Next to cycloadditions, other pericyclic reaction mecha- esters, which are well known as end groups in RAFT-type con- nisms can also establish a rapid reversible adduct formation trolled radical polymerisations, as reactive dienophiles that between relatively stabilized (long living) reactants. The can reversibly add to cyclopentadienyl-functionalised (Cp) Alder–ene reaction is a classic example of an alternative peri- polymers in a swift hetero-Diels–Alder reaction (Scheme 6c).48 cyclic bond forming process, and in 2014, Winne, Du Prez The authors also developed straightforward methods to incor- and coworkers reported dynamic polymer systems based on porate reactive Cp-moieties in polymers and studied their the reversible ene reaction between an indole and a triazoli- dynamic bond formation extensively. The forward hetero- nedione (TAD).51 Whereas TADs are mostly known as dieno- Diels–Alder reaction between Cp and thioesters proceeds philes in swift and mostly irreversible hetero-Diels–Alder within a matter of minutes at room temperature. With typical reactions, they also act as highly reactive enophiles.52 When − reaction free energies in the range of −35 to −45 kJ mol 1, the combined with indoles, Baran and Corey found that TADs degree of bond formation is markedly more pronounced than form a very stable Alder–ene adduct (Scheme 6d), which is in the two previously described Diels–Alder reactions, rapidly nonetheless reversible at high temperatures (>100 °C).53 This proceeding to full conversion at room temperature, but the reversible adduct formation was investigated in a macro- bond formation can still be significantly reversed at higher molecular context by Winne and Du Prez, giving a stepwise temperatures through a retro-Diels–Alder process. Like all dissociative MNR pathway with an exceptionally high free − spontaneous bond forming processes, this reversible Diels– energy of reaction in the range of 60–70 kJ mol 1.54 Alder association should involve opposing entropic and enthal- Nevertheless, the low reaction barriers that are associated pic driving forces, with the entropic factor normally favoring with the forward TAD-indole reaction, owing to the high reac- debonding, and the enthalpic factor favoring bonding to over- tivity of TADs, results in a still feasible barrier for the back- come the entropic penalty associated with linking two mole- ward debonding reaction. Interestingly, in light of these cules. At high enough temperatures, the enthalpic bonding thermodynamic parameters, significant debonding can

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. preference of all reversible cleavage reactions will diminish, theoretically not be achieved in such systems, even at highly revealing the entropic debonding preference according to: elevated temperatures. However, the small, transient fraction of debonded free TAD chain ends can kinetically still be ΔH° R ln K ¼ þ ΔS° quite relevant, and indeed significant network reorganis- T ation, full stress relaxation and flow can be achieved in these and a shift in the equilibrium towards a less densely cross- networks, without necessitating a net depolymerisation or linked material should result. The upper limit of the dis- debonding.55 sociation equilibrium constant is athermal and will be deter- Many dissociative stepwise covalent bond exchange chem- mined by the difference in entropy between the two states. A istries (reversible additions) involve pericyclic reactions, but This article is licensed under a full macroscopic reversal of any bonding equilibrium can only also simple (ionic) addition reactions have been investigated be achieved by extreme dilution or by evaporation of the in this context. For example, the reaction between isocyanates binding partners. and (hindered) nucleophiles has long been known to be a ther- Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. In their studies of reversible Cp-thioester Diels–Alder cross- moreversible process and has also found industrial appli- linking,48,49 Barner-Kowollik and coworkers found that next to cations in polymers through the development of so-called the effects of chemical substitution pattern on the diene and blocked isocyanate reagents.56 In 2014, Cheng and coworkers dienophile (e.g. more and less electron-rich dithioesters), the introduced reversible bonds between isocyanates and hindered bonding-debonding equilibrium can also be strongly depen- (secondary) amines as a dissociative stepwise MNR process for dent on the exact macromolecular architecture, demonstrating polymers with dynamic urea cross-links (Scheme 6e).57 The significant changes in the reaction entropy as a function of the process involves a simple proton transfer next to the urea length of polymer chain fragments that separate the dynamic bond cleavage event, so that neutral fragments can be formed. cross-links.49 Using a theoretical model, combined with experi- Although isocyanates can be prone to irreversible side reac- mental observations, highly significant differences in the tions such as hydrolysis or allophanate formation, their avail- bonding/debonding equilibria were found and related to the ability as common monomers allows for a straightforward effect of chain length on the reaction of otherwise chemically introduction of dynamic bonds into many polymer matrices. identical reaction partners. These results are in line with those The reaction free energy here also strongly depends on the of previous theoretical studies on entropy effects in hydrogen nature of the used hindered amine and can thus be varied or bond associated polymers studied by Stadler,50 and again tuned over a wide range, from mildly exothermic to highly underline the highly significant but far from straightforward exothermic. effects that the macromolecular architecture can have on the A final example we want to draw attention to in this thermodynamics of a dynamic bond exchange process. In this section is the use of reversible Michael additions of thiols to light, it can be expected that similar effects can be found also conjugated, electrophilic alkenes. These are interesting on the reaction kinetics of MNR reactions, and such studies dynamic linkages as they are in fact widely used in polymer could be of great value for our understanding of dynamic materials as a thiol-Michael click reaction,58 and can be polymer systems. readily prepared from for example acrylate and thiol mono-

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Perspective Polymer Chemistry

mers. In this context, the formation of the thioether bond is In 2015, Montarnal and Drockenmuller reported the syn- usually considered as an irreversible ‘click’-like linkage. thesis of a polyionic polymer network incorporating However, the reversible nature of this Michael reaction (in N-alkylated triazolium iodide moieties as cross-links basic medium) has been known for a long time in organic (Scheme 7a), conveniently prepared using Huisgen-type azide chemistry.59,60 In 2016, Schubert and Hager implemented alkyne click-like chemistry and an alkylating cross-linker for this dynamic thio-Michael addition for the design of healable the resulting nucleophilic triazole species.63 The obtained net- polymer networks (Scheme 6f).61 They used a highly reactive, works could undergo stress relaxation through an associative

but stabilised malononitrile-derived Michael acceptor. In the SN2-type exchange that constitutes a transalkylating reaction of presence of a mild base, the malonitrile can be readily depro- triazoles as nucleophilic species. Interestingly, however, such tonated and a very rapid dynamic equilibrium establishes networks were later actually shown to be dominated by a more itself from which actually neither the Michael adduct or the complex dissociative stepwise MNR pathway, wherein the reaction partners can be isolated in pure form, due to their iodide counterion induces a temporary decross-linking via a unavoidable interconversion. The observed rapid exchange highly reactive alkyliodide chain end. In fact, this is a clear also prompted them to study the dynamics on a material example of a small molecule-mediated dissociative exchange level. One of the main issues here, was the dynamic nature of via a reversible condensation process (cf. Scheme 3). In an normal acrylate-based thioether linkages under the basic con- elaborate but elegant in-depth follow-up study, the rheology

ditions, leading to the release of H2S over time, and thus to data together with data on network composition obtained material degradation. Because of the widespread utility of from XPS measurements and statistical calculations, un- thiol–ene chemistries, their base-promoted reversibility ambiguously confirmed an iodide-mediated stepwise disso- seems to be a promising avenue for further research, ciative MNR pathway, involving the intermediate reformation although it is clear that the design of monomers will need to of the alkyl iodide cross-linkers, and Montarnal and take into account their longer-term stability issues under the Drockenmuller could actually exclude a kinetically significant

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. applied conditions. In simultaneous studies by Konkolewicz SN2-type concerted MNR pathway, at least at temperatures up and coworkers,62a the reversible thiol-Michael addition to 150 °C.64 This result points towards the pitfalls that can be exchange has also been investigated in more common acry- faced when considering MNR mechanisms in dynamic late-based networks, shown to be operative at higher tempera- covalent networks, and that care must be taken before con- tures in the absence of a base. This work has also led this clusions about the nature of MNRs are drawn. group to reinvestigate the dynamics of the thiol-Michael In 2017, Winne and Du Prez reported a related trans- addition using elegant small molecules.62b alkylation chemistry, applicable to polythioether networks For other notable dynamic covalent chemistries that have (Scheme 7b).65 Here, the alkylating cross-links are provided been implemented in dissociative stepwise MNR-type polymer by sulfonium sulfonate ion pair. Interestingly, using a non- This article is licensed under a networks, we refer to more specialized reviews on these nucleophilic counterion for the sulfonium ion cross-links topics.5,7,11,12 was found to be important for material properties. Alkyl halides did not give satisfactory results, likely caused by Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. their higher nucleophilicity in SN2-type dealkylation reac- Concerted rearrangements tions. The chemistry of the MNR bond exchange implemented here is based on the classical SN2-chemistry By far the least common type of MNR mechanisms are single underlying cationic ring opening polymerisation of cyclic step, concerted rearrangements, where no net bonds are formed thioethers, as introduced to polymer chemistry by or broken under any circumstances (Scheme 2a). In fact, to our Goethals.66 Via alkylation of thioether networks, a strategy knowledge, only two dynamic covalent chemistries for ‘vitrimerisation’ of polythioether thermosets into implemented in polymer networks have been suggested to go dynamic networks via alkylation thus seems viable. Very through a concerted bond exchange pathway (Scheme 7). recently, Guo and Zhang introduced this sulfonium-sulfide transalkylation chemistry in natural rubber vulcanisates, achieving an eﬀective recycling strategy for these classical materials via alkylation and subsequent transalkylation.67 The possibility of competing leaving group-mediated dealkylation (reversible condensation) has so far not been investigated in these systems, but this should be done in light of the recent findings of Montarnal and Drockenmuller. As it stands, this chemical platform is the only one that has a ‘theoretically ideal’ energy profile for the establishment of vitrimeric properties. As concerted MNR materials can have interesting properties, related to their strictly constant cross- link density, this certainly seems like an area within dynamic Scheme 7 Examples of (possible) concerted MNR-based materials. polymer systems that deserves more attention.

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Small molecule-mediated dissociative stepwise rearrangements In addition to the three above discussed generalised categories of MNR pathways (Scheme 2), the small molecule-mediated dissociative stepwise MNR pathways (Scheme 3) also deserve some discussion with specific examples. Although superficially, a very similar situation arises here as in normal stepwise dissociative MNRs, all bond cleavage events are strictly conditional on the presence of a small molecule polymer additive. Association of polymer chain segments is not required for the rearrangement reaction to occur, but rather an association between a cross-link and a (more mobile) small molecule additive must occur (a retro-polymer condensation step) before a chain can fragment into two new reactive chain ends. When only a substoichio- metric amount of the decross-linking additive is present (i.e. a catalytic amount of the transfer group), the network cannot undergo extensive depolymerisation, and will remain mostly intact or permanent, while it can rearrange its structure through intermediacy of the decross-linked chain ends. In terms of approaching the dynamic network criteria for vitrimer polymers, this category can thus be also quite interesting from

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. a practical point of view. In any case, this type of process is inherent to almost all associative MNR systems, as a low molecular weight, monofunctional additive that has the same reactiv- Scheme 8 Small molecule mediated dissociative stepwise MNRs. ity as that of dynamic link will install such pathways at least as side reactions, or through mobile network defects. As an early example of a dynamic covalent polymer imines and hydrazones can be expected to also undergo network, incorporating fast breaking and forming of bonds associative addition/elimination exchanges in the presence of through a reversible hydrolysis, Leibler and coworkers pointed pendant amine-functionalities. However, also under ‘wet’ con- out already in 1990 that polysaccharide hydrogels, covalently ditions the extent of network dissociation can be avoided or at This article is licensed under a cross-linked by borax-derived boric esters, are self-healing at least controlled by limiting the availability of water. Indeed, in 68 room temperature (Scheme 8a). Although these hydrogels 2014, Zhang and coworkers reported reversible polyimine net- may be fragile towards hydrolysis at higher temperatures, the works based on imine hydrolysis,71 that could also undergo Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. exchanges at lower temperatures are very fast and most bonds associative exchange under dry conditions, nicely demonstrat- remain formed but are highly dynamic, showing an interesting ing a system that can conditionally switch between prevalent compromise situation that can lead to emerging properties associative and dissociative pathways. Such dynamic polyimine such as self-healing. resins are currently being developed industrially by a start-up Next to bor(on)ic esters, one of the best studied polymer company.72 systems to show a dissociative reversible polycondensation Very recently, Konkolewicz reported another example of a pathway, are polyimine and poly(acyl)hydrazone based net- reversible condensation MNR pathway operating in dynamic works. These networks are formally formed by condensation polymer networks, by implementing transalkylation reactions of of aldehydes or ketones with amines or hydrazides. The amines, via temporary de-cross-linking by adventitious bromide – carbon nitrogen double bond spontaneously forms by mixing ions as good nucleophiles (Scheme 8c).73 These polyionic net- aldehydes with a suitable amine-type nucleophile, albeit with works undergo an SN2-type cleavage at the benzylic position, a limited thermodynamic driving force, which is helped by the releasing a highly reactive benzyl bromide intermediate chain exclusion of a molecule of water that can be used to drive the end. This can in turn react via an SN2-process with other free equilibrium. As this condensation reaction is inherently revers- amines. This is thus in fact a very similar mechanism to that 69 ible, an endothermic hydrolysis can be facilitated by heating, found to be operative in polyionic alkylated triazole networks or by increasing the concentration of water. The thermal mal- developed by Montarnal and Drockenmuller (Scheme 7a). leability of these networks is typically found to be highly conditional on the presence of water. In 2005, Lehn and coworkers introduced their acylhydra- Dissociative vitrimers? zone-based dynamic covalent chemistry platform in the synthesis of dynamic polymer networks that rely on a reversible In the recent literature, there has been some confusion over hydrolysis reaction (Scheme 8b).70 In the absence of water, the definition, properties and conditions that define vitrimer

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Perspective Polymer Chemistry

materials.64 Several polymer networks that clearly undergo dissociative exchange reactions, have been shown to possess all the physical properties that are expected for vitrimers: they are insoluble in good solvents, but flow upon heating with a very gradual thermal viscosity profile (linear Arrhenius-like dependence of the viscosity as a function of temperature and no sol– gel transition). Thus, the question arises: “is an associative exchange reaction an absolute prerequisite for a polymer network to show vitrimer behavior?” The answer to this question, in our opinion, is quite context-dependent. One of the major characteristics of vitrimers is their Fig. 1 Two dissociative stepwise MNR-based networks can have the gradual viscosity profile, further showing a temperature depen- same bond exchange rate, but a diﬀerent temperature response in terms dence in line with that of the chemical exchange reaction, with of network integrity. an apparent link between mechanical and chemical activation energies. Above a certain temperature, the rheological behavior of vitrimers, as assessed by mechanical stress relaxation experiments, will be dominated by the chemical exchange bond between chain ends). It should be clear that both net- kinetics. In other words, the ‘rate determining step’ in chain works will theoretically show exactly the same rate of bond mobility will be a chemical reaction and will show the same exchanges at all temperatures, as the forward activation barrier temperature dependence, following an Arrhenius law. It is the same, while their viscosity profiles and macroscopic pro- should be noted that this type of behavior is not limited to perties will be quite diﬀerent. Network integrity can in fact be vitrimers or to polymer networks, as chemical stress relaxation maintained in such dissociative networks if the relative abun-

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. can become the dominant phenomenon in the rheology of a dance of the open forms, as dictated by the reaction equili- polymer through a variety of factors under certain circum- brium constant, is kept low (e.g. <5% decross-linking). A sig- stances. Also within vitrimers, this Arrhenius-like behavior is nificant amount of debonding can only be expected at temp- not guaranteed under all circumstances, for example when the eratures that shift the equilibrium to the endothermic side.

temperature approaches Tg and the onset of vitrification puts This situation arises when the available thermal energy is additional limits on chain mobility.27 enough to overcome the enthalpic driving force of the bond In stepwise dissociative MNR pathways (Schemes 2c and 3), formation, in favor of the entropically more favored debonded de-cross-linking can certainly be expected to significantly states. When ΔH for debonding is quite large, the temperature affect viscosity upon heating. However, covalent exchange reac- necessary to shift the equilibrium will often be above the This article is licensed under a tions can become kinetically relevant at temperatures where degradation temperature of the polymer matrix. As a result, net de-cross-linking is still very low and does not affect the vis- the polymer network that is rearranging through a dissociative cosity. In other words, in situations where cross-links are pathway will show a constant cross-link density or at least Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. reformed as fast as they are broken, there can be no significant cross-link densities where loss of network integrity is build-up of free chain ends. Nevertheless, the underlying negligible. chemical exchange reactions can become the dominant stress For many practical purposes, it can be understood from the relaxation mechanism in such dissociative networks. above example that a dissociative network might show charac- Chemically controlled stress relaxation or non-fluctuating teristics and properties of vitrimers, providing similar advan- degrees of connectivity is thus not an exclusive feature of tages for processing and reprocessing. This has indeed been associative MNR polymer systems. Dissociative exchange net- reported in the literature in a few recent examples, where works should in theory always show decross-linking, but the Arrhenius-like viscosity profiles have been observed, together difference in free energy between the bonded and debonded with a constant elastic modulus, indicating a permanent states of the networks can prevent a sol–gel transition from network over the probed temperature range.63 However, an happening at a reasonable temperature (e.g. <300 °C). This important difference between associative vitrimer materials situation will be clarified by a hypothetical example (Fig. 1). and dissociative vitrimer-like materials will be their intrinsic When considering networks that undergo bond exchanges resistance to dissolution in a good, non-reactive solvent. In a through a stepwise dissociative MNR pathway, the reaction dissociative network that swells in a good solvent, the debond- enthalpy of the reversible bond forming step can be con- ing equilibrium is expected to readily shift towards the sidered as a crucial factor. For the sake of discussion, a situ- debonded state by the dilution effect, and complete dis- ation is sketched in Fig. 1 for two dissociative networks that solution is in principle possible. In a permanent network, or have exactly the same activation energy barrier for the debond- more precisely in a vitrimer, the overall network connectivity ing step. In one case, the backward reaction barrier for rebond- in the sol fraction and within the remaining network will stay ing is assumed to be relatively small (fast formation of bond the same at all temperatures. As found in theoretical studies

between chain ends), while in the other case this barrier is on a model AfB2 vitrimer, the sol fraction of the network will assumed to be much higher (much slower reformation of the depend on solvent concentration, but not on temperature.22a

Polymer Chemistry Perspective

Some specific polymer backbones, however, can undergo a context of chemically reactive polymer materials, it should be thermally triggered depolymerisation reaction, as discussed realized that the precise nature and exact architecture of the for the classical PDMS networks that can undergo alkoxide- polymer matrix, and possible polymer additives or even minor mediated reversed ring opening polymerisation (cf. Scheme 5), impurities can all have a drastic influence on material pro- next to network bond exchanges, leading to considerable, and perties that can only be understood by taking into account thermally dependent sol fractions, even though the number of mechanistic aspects. The findings by Barner-Kowollik and co- bonds stays the same. Such features are linked to the macro- workers described earlier for the dynamic thioester Diels– molecular make up and synthesis, and the likelihood of cycla- Alder polymerisation,49 indeed point towards a significant tive cleavages within the network. An associative mechanism interdependence of monomer structure and the reaction thus seems to be required for all vitrimer properties but is not entropy for debonding, a result that should be taken as a always suﬃcient. A further exception or important caveat that warning against liberally comparing the situation of bond should be made here, is for the dissociative networks that exchange reactions between small molecules (model com- require mediation by an additive or an associated catalyst to go pounds) in solutions and those taking place inside polymer to the decross-linked state (cf. Scheme 3). For such reversible network segments. condensation-type networks, the extent of decross-linking that Chemical reactions can be accelerated by the presence of a can be attained at any temperature will be conditional on the molecular species that acts as a catalyst, or by the specific relative amount of the reactive small molecule additive or cata- nature of a solvent. Thus, the rheological behavior of dynamic lyst. If this amount is low, only a small fraction of polymer seg- covalent networks will certainly depend on the presence and ments can be in the dissociated state at any point in time, and concentration (or availability) of such catalytic species in the these networks undergo dissociative MNRs, but they should polymer matrix.74 This is a straightforward example of how a nevertheless be permanent, and are also expected to be resist- small change in the formulation of a polymer network can ant to dissolution by a good solvent, as the equilibrium cannot result in an important change in its mechanical properties. In

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. be significantly shifted to a non-bonded state. the design and study of dynamic covalent polymers, such For demonstrating the vitrimeric properties of a given effects should be expected, scrutinized and experimentally polymer network, and thus to show its non-fluctuating cross- probed. link density, stringent solubility tests should always be per- Often, in literature dealing with vitrimers, a comparison is formed, which is unfortunately not always reported when new made between chemical and mechanical relaxation times. ‘vitrimer’ chemistries are disclosed. In fact, we predict that This can be instructive but also unwarranted. It is indeed many reported dissociative ‘vitrimers’ based on a stepwise dis- impossible to exactly mimick the conditions of exchange reac- sociative MNR mechanism are likely to be readily soluble in tions within a macromolecular network in a solution with low good solvents, although it may be difficult to find a good MW reagents. A material put under mechanical stress is an This article is licensed under a solvent for some cases. out-of-equilibrium situation that is not directly comparable to In summary, the distinction between vitrimer (permanent) a freely equilibrating chemical reaction between similar dynamic networks and thermoplastic (reversible) dynamic net- species in solution. One should thus not expect a clear corre- Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. works can become a highly academic one, as also dissociative lation between the reaction rates observed in chemical model networks can show very high resistance to dissolution in sol- experiments and in dynamic polymer networks. Indeed, many vents. Nevertheless, the interplay between macroscopic reports have found a significant difference in relaxation times, material properties and chemistry remains a fascinating area and any observed correlations are mostly qualitative. for further research and, as is expected for chemical reactivity, Differences have not only been observed in absolute relaxation dynamic polymer properties can be complex and are highly times, but also in their temperature dependence (i.e. activation context dependent. Before going to our conclusions, we would energy). Small molecule model studies are thus necessary to like to underline this final point in a dedicated final section, understand polymer dynamics, but they are also far from as it is something that is often overlooked or neglected in the sufficient. Qualitative trends in reactivity observed on small context of materials chemistry. molecule models are probably the most important information that can be gained from such studies. On the other hand, as there is a lack of good characterization tools for studying Context-dependent mechanical chemistry in networks, studying models will obviously remain properties critical for our understanding in this area. Another aspect that is often overlooked in considering the Whereas the chemical structure is an innate and absolute mechanism of polymer rearrangements, is that a given MNR feature of a molecular species, its chemical reactivity is not. reaction can theoretically always proceed via multiple different Care should thus be taken in considering the properties of a reaction pathways, and that either of those pathways can be rearranging polymer network. Chemical reactivity is a highly promoted or inhibited by the specific context of a material relative phenomenon, and typically shows strong context matrix. Care should thus be taken not to consider a particular dependency: reagent purity, solvents, order of mixing, and type of chemistry as a purely associative or purely dissociative additives can all have a critical influence on reactivity. In the one! For the seminal example of Leibler’s polyester poly-

Perspective Polymer Chemistry

receiving indoles. Even though debonding and rebonding rates are fast in these systems, macroscopic polymer network dynamics of TAD-indole based dynamic polymer networks is not observed unless an excess of indoles, relative to the amount of TADs, is present throughout the network structure.55 As stated before, the macromolecular context can also drastically influence polymer dynamics and chemistry. For example, matrix incompatibility effects can lead to microphase separation, and completely change macroscopic properties if Scheme 9 Known competing pathways for ester-hydroxyl exchange the exchangeable groups aggregate into domains.79 reactions. Finally, as made clear by the example poly-ionic triazolium networks (cf. Scheme 7a),63,64 the difference between concerted and stepwise pathways can be hard to determine experi- hydroxyl vitrimer networks (cf. Scheme 4a),14,15,23 for example, mentally. Accurately and effectively modeling MNR reactions is it should be taken into account that transesterifications can therefore predicted to be an outstanding goal for theoretical actually also proceed via a dissociative pathway, for example chemistry. The influence of solvatation on potential energy with the intermediacy of ketenes as high energy, debonded surfaces of small molecule reactions is already a challenging neutral intermediates (Scheme 9, bottom pathway).75 That the computational task. Theoretical approaches that can include chemistry of epoxy-based transesterification vitrimers is more the influence of a macromolecular network matrix may thus – convoluted than anticipated was also recently highlighted by be very challenging.22a,80 82 Williams.76 Similar plausible alternative reaction pathways can During the last two decades, a great effort has been devoted

Creative Commons Attribution-NonCommercial 3.0 Unported Licence. in fact be considered for all known stepwise associative MNR to the design and synthesis of dynamic polymer networks. As chemistries. The fact that they are not observed as prevalent we discussed throughout this paper, much progress has been pathways on small molecule models in solution phase, is not a made to introduce versatile and robust but at the same time guarantee that the same will be true in a rearranging polymer reactive organic building blocks into polymer chemistry, network under mechanical stress. enabling control over the topology and molecular dynamics of A very concrete example of two competing exchange path- a large set of polymer matrices. Comparatively much less is ways for the same exchange reaction, was observed in our own known about the material and rheological properties of research of vinylogous urethane vitrimers, where it was found polymer networks that incorporate such dynamic covalent that very similar polymer networks, containing the same chemical linkages. Even in the absence of side reactions, the This article is licensed under a amount of chemically reactive moieties, can have two compet- link between MNR pathways and kinetics and macroscopic ing exchange pathways, as confirmed by a marked difference properties can be quite complex and deserves closer attention. in temperature dependence (i.e. activation energy).77 In these For dissociative mechanisms, some guidelines could come Open Access Article. Published on 16 10 2019. Downloaded 2021-10-02 12:36:15. vitrimers, we found that the mechanical relaxation time, as from the numerous studies of so-called associating polymers well as the dominant exchange mechanism, critically relies on for which a rich spectrum of behavior has been identified small changes in formulations, additives, and also the overall both experimentally and theoretically, taking into account polymer architecture, resulting in differences in orders of mag- factors such as temperature, number of temporary bonds, nitude for the observed relaxation times, within chemically all their life-time, macromolecular structure and stoichiometry of but identical polymer matrices. Very recently, Guerre and Du the network.18,20,83 Various “sticky” reptation, Rouse or Prez even found two distinct MNR mechanisms operating each dynamic aggregate models have been developed for these as the dominant process within the same material, but at systems to describe rheology or self-healing properties.84 Such different temperature intervals, showing a dual temperature models rely on the transient nature of sticky associations response.78 between chains, where the life-time of any bond can be As another example of context dependence in dynamic expected to be short. Even in such systems, strong context covalent polymers, Winne and Du Prez have found that the dependence will already emerge. For example, when associat- dynamic bonding and debonding of TAD-indoles (cf. ing groups (stickers) are diluted, their rapid opening and Scheme 6d),51 the stoichiometry of indoles and TADs in closing will most of the time recombine the original partners. the network plays a crucial and interesting role. When there is In this context, rapid opening and closing cannot yield a fast a 1 : 1 equivalence between the two reactive moieties, full network topology reorganization or, from a mechanical point cross-linking or curing can be achieved owing to the high of view, to flow or stress relaxations.20 Thus, in the context of exothermicity of the reaction, and no free indole or TAD MNRs that involve longer lived bonds, exchanging via either remains in the network. As there can be no significant build- dissociative or associative MNR pathways, an even more up of free chain ends in this case (very fast reformation of complex relationship to mechanical properties can be chain ends, cf. Fig. 1a), the reaction rate for rebonding to expected. As discussed above, in principle, for some stoichi- another indole will rely mainly on the availability of such ometries, networks with dissociative MNR pathways could

Polymer Chemistry Perspective

behave as networks with exclusive associative pathways, as net- Conﬂicts of interest decross-linking does not occur.85 Also, it should be stressed that in polymer matrices, reactive groups and catalysts interact There are no conflicts to declare. with the macromolecular network and will exhibit their own dynamics, which may be diﬀerent from that in ‘free’ solutions. Therefore, understanding how such interactions and motion References dynamics in the network combine and influence the emergence of material properties is at this time an interesting and 1 R. J. Wojtecki, M. A. Meador and S. J. Rowan, Using open research question. the dynamic bond to access macroscopically responsive structurally dynamic polymers, Nat. Mater., 2011, 10(1), 14. Conclusions 2 N. Roy, B. Bruchmann and J. M. Lehn, DYNAMERS: dynamic polymers as self-healing materials, Chem. Soc. Dynamic bond interchange between portions of macromolecu- Rev., 2015, 44(11), 3786–3807. lar chains is a fascinating dynamic rearrangement process in 3 B. J. Blaiszik, S. L. Kramer, S. C. Olugebefola, J. S. Moore, polymer chemistry, which, despite its potential, has so far N. R. Sottos and S. R. White, Self-healing polymers and found limited industrial applications. However, academic and composites, Annu. Rev. Mater. Res., 2010, 40, 179–211. industrial interest in chemistries that allow such intrinsic reac- 4 J. A. Syrett, C. R. Becer and D. M. Haddleton, Self-healing tivity within polymers has surged in the last decade, with the and self-mendable polymers, Polym. Chem., 2010, 1(7), 978– first applications already finding their way.72 This has been 987. mostly brought about by academic groups interested in the 5 T. Maeda, H. Otsuka and A. Takahara, Dynamic covalent design of functional polymer materials with novel useful pro- polymers: reorganizable polymers with dynamic covalent perties. As ever more dynamic covalent chemistry platforms 34 – Creative Commons Attribution-NonCommercial 3.0 Unported Licence. bonds, Prog. Polym. Sci., 2009, (7), 581 604. are being explored within a macromolecular context, it is 6 M. A. C. Stuart, W. T. Huck, J. Genzer, M. Müller, C. Ober, becoming clear that studies so far have still been mostly M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, scratching at the surface, and that a fundamental understand- M. Urban, F. Winnik, S. Zauscher, I. Luzinov and S. Minko, ing of what is governing the dynamics of bond interchanges is Emerging applications of stimuli-responsive polymer lacking at this time. materials, Nat. Mater., 2010, 9(2), 101. We hope that this perspective has put the various funda- 7 C. J. Kloxin, T. F. Scott, B. J. Adzima and C. N. Bowman, mental issues that need to be addressed in a logical frame- Covalent adaptable networks (CANs): a unique paradigm in work, and that it can provide both inspiration and guidance cross-linked polymers, Macromolecules, 2010, 43(6), 2643–

This article is licensed under a for future studies in this fascinating area at the interface of 2653. polymer science and organic chemistry. 8 C. J. Kloxin and C. N. Bowman, Covalent adaptable net- Mechanistic considerations in organic reactivity are far works: smart, reconfigurable and responsive network

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Perspective Polymer Chemistry

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